This paper describes an effect of fluctuation in fiber orientation on Young’s modulus of the so-called green composites. The composites were reinforced with slivers of high-strength natural fibers extracted from plants named curaua. Then a surface optical micrograph of the composites with the fiber fluctuation was obtained. The micrograph was divided into many fine segments, and the fiber orientation angle in each segment was measured. Then, a new concept which takes account of the fiber orientation angles as a probability distribution, was proposed for prediction of the Young’s modulus. The results showed that the predicted Young’s moduli were in a good agreement with the experimental results.